[BUGFIX] Set uid of BE_USER mock in functional tests

Datahandler functional tests lack proper initialization of the
BE_USER mock used.

Set a 'uid' for the mock in order to get tests working again.

Resolves: #54741
Releases: 6.2
Change-Id: I20cccf8d3622b2c63d6b7c9f07c0e2d238bea7b9
Reviewed-on: https://review.typo3.org/26642
Reviewed-by: Oliver Hader
Reviewed-by: Markus Klein
Tested-by: Markus Klein